all details have been provide in https://jira.mongodb.org/browse/SERVER-36512 but this topic focus on 2 new found issues 

I am confused with Nick's feedback. If the Nick's answer is right, then where are the down major members? because of the replset has 1 primary 2 secondaries and 1 arbiter, only one secondary was unreadable when the issue is found.

I did more investigation on the mongod.log find another 2 issues: 

  issue: primary still contact removed 3 members, fixed by restart mongod service

  issue: primary did not step down if it still thinks the replset has 6 data bearing members and 1 arbiter and 4 data bearing members are down.

 
secondary : 172.31.54.204 (primary when the issue happen on Aug 6)
arbiter : ip-172-31-5-208 (was 3.4.7 when the issue happen on Aug 6)
old member 3.4.7 : ip-172-31-12-59 (removed from replset July 23)
old member 3.4.7 : ip-172-31-20-52 (removed from replset July 23)
old member 3.4.7 : ip-172-31-46-24 (removed from replset July 23)
secondary : ip-172-31-66-130 (unreachable when the issue happen on Aug 6)
primary : ip-172-31-82-157 (secondary when the issue happen on Aug 6)
secondary : ip-172-31-67-188 (added afer the issue happen)

All 3.6.6 now